The present invention is in a process of phosphating galvanized surfaces, particularly of galvanized steel, by a treatment with aqueous phosphating solutions which contain zinc ions, phosphate ions, additional layer-forming cations and accelerator, and to the use of that process for the treatment of galvanized steel strip, which treatment is optionally followed by the application of paint or a preformed organic film.
From Published German Application 21 00 021 it is known to treat metal surfaces with phosphating solutions which contain nickel as an essential cation. In that case, phosphate layers which in addition to zinc contain substantial amounts of nickel as a cation are formed on zinc surfaces. Such layers have a very high resistance to corrosion, particularly when they have been afterrinsed with the usual afterrinsing liquors, which contain Cr(VI)-Cr(III). The layers also constitute an excellent primer for paint to be applied to the strip. But that process has the disadvantage that the conversion layer which has been formed has a relatively dark, almost black, color, which is not attractive and gives rise to color problems in case of an application of bright and white paints.
Published German Application 32 45 411 describes a process of forming zinc phosphate layers on electrolytically galvanized steel. An advantage of that process resides in that a mass of less than 2 g/m.sup.2 hopeite is obtained within a short time so that the resulting galvanized and phosphated strip can subsequently be welded. A disadvantage resides in the restriction to electrolytically galvanized surfaces. Since strip which has been hot dip galvanized has a low reactivity to the phosphating solution, it is not possible, in most cases, to form the desired layer in a desirable short treating time.
It is also known that phosphate layers which mainly consist of hopeite (Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O) are inferior in its properties in comparison to phosphate layers which mainly consist of phosphophyllite (Zn.sub.2 Fe(PO.sub.4).sub.2.4H.sub.2 O) (K. Wittel: "Moderne Zinkphosphatier-Verfahren-Niedrig-Zink-Technik", IndustrieLackierbetrieb, 5/83, page 169, and 6/83, page 210). However, commercially feasible processes for forming phosphophyllite layers on zinc surfaces are not yet known.
It is an object of the invention to provide for the phosphating of galvanized surfaces a process which can be used with equally good results with zinc coatings which have been applied electrolytically or by a hot dip process and which will result in a formation of bright, almost white coatings. It is also an object of the present invention to provide a process which results in the formation of phosphate layers of less than 2 g/m.sup.2 and which will provide a good bare corrosion protection, i.e., a corrosion protection solely effected by the phosphate layer and can be used as a primer for paint and preformed organic films. It is a further object of the present invention to provide a process which will result in the formation of coherent coatings within a short time.